1. Field of the Invention
The present invention relates in general to a display device, more specifically, to an effective technique to be applied to a chip-on-glass (COG) type display device.
2. Description of the Related Arts
Thin Film Transistor (TFT) type liquid crystal display devices are broadly used for laptop computers or TV panels. These liquid crystal display devices include respectively a liquid crystal display panel, and a semiconductor chip mounted with a drive circuit (e.g., a gate driver circuit or a source driver circuit) for driving the liquid crystal display panel.
Such a liquid crystal display device adopts a chip-on-glass (COG) method in which sleeve semiconductor chips are mounted directly on one of a pair of substrates that constitute a liquid crystal display panel. A variety of semiconductor chip mounting schemes based on this COG method have been suggested and put to practical use. One of them is ACF mounting in which an anisotropic conductive film (ACF) is used to mount a semiconductor chip. According to the ACF mounting, wiring lines (or simply wires) formed at one of a pair of substrates constituting a liquid crystal display panel are electrically and mechanically connected to a bump electrode formed on the principal surface of a semiconductor chip by way of an anisotropic conductive film. One example for obtaining the anisotropic conductive film is to disperse and mix a plenty of conductive particles into an epoxy based thermosetting insulation resin.
A COG type liquid crystal display device is described in JP-A-2002-258317.
In the ACF mounting, an ACF is interposed and heated between one substrate of a liquid crystal display panel and a semiconductor chip 170 to compress the semiconductor chip 170, and conductive particles 21 in the ACF are inserted between wiring lines of the substrate and bump electrodes 4 of the semiconductor chip 170 to connect both (the wiring lines and the bump electrodes) electrically and mechanically. It is therefore necessary to align the bump electrodes 4 of the semiconductor chip 170 to a uniform height, namely, planarizing the plural bump electrodes. However, as shown in FIG. 10 (this is a drawing for explaining a semiconductor chip warpage phenomenon in a related art), the principal surface of the semiconductor chip 170 (i.e. the bump formation surface where bump electrodes 4 are formed) is warped in a convex manner. In result of the warpage, a bump electrode 4a arranged in the vicinity of the center for the longitudinal direction of the semiconductor chip 170 has a different height from bump electrodes 4b1 and 4b2 that are arranged in the vicinities of end portions (an end portion of one side Sd1 and an end portion of the other side Sd2), respectively. Since the bump electrodes 4 with nonuniform heights cause connection failure between the wiring lines on a substrate and the bump electrodes of a semiconductor chip, or unstable connection resistance therebetween (between the wiring lines and the bump electrodes) and degrade the reliability of a liquid crystal display device, there is a need to develop a technique to resolve these deficiencies.